Raw materials comprising polymer compounds each have characteristic gas permeability for the respective raw material. Based on properties thereof, a desired gas component can be separated, by allowing selective permeation, by means of a membrane constituted of a specific polymer compound. As an industrial application embodiment of this gas separation membrane, study has been conducted for separating and recovering carbon dioxide from a large-scale carbon dioxide source, in a thermal power station, a cement plant, a shaft furnace in a steel plant, or the like, in relation to the issue of global warming. Then, this membrane separation technique attracts attention as a solution to environmental issues, which is capable of allowing achievement with a relatively small energy. Meanwhile, natural gas or bio gas (gases generated by fermentation and anaerobic digestion, for example, of excreta of organisms, organic fertilizers, biodegradable substances, polluted water, garbages, energy crops, and the like) is mainly a mixed gas of methane and carbon dioxide. Study has been made so far for a membrane separation method as a means for removing an impurity, such as carbon dioxide, therein (see Patent Literature 1 and Patent Literature 2).
Specifically, study has been made for celluloses or polyimides as a raw material in purification of a natural gas. However, the membrane is plasticized under high pressure conditions and high carbon dioxide concentration in an actual plant, and a lowering of separation selectivity due to the plasticization has become a problem (see Non-Patent Literature 1, pp. 313-322; and Non-Patent Literatures 2 and 3). In order to suppress plasticization of the membrane, introduction of crosslinked structure into a polymer compound constituting the membrane is known to be effective, and research has been continued on a separation membrane using a polyimide resin (see Non-Patent Literature 1, pp. 3-27). Specific examples of utilization of a membrane having a crosslinked structure for the gas separation membrane include those described in Patent Literature 3, and Non-Patent Literatures 4, 5 and 6.